salinomycin: anti-tumor activity in a pre-clinical colorectal cancer … · 2020. 1. 11. ·...

RESEARCH ARTICLE

Salinomycin: Anti-tumor activity in a pre-

clinical colorectal cancer model

Johannes KloseID1*, Stefan Trefz1, Tobias Wagner1, Luca Steffen1, Arsalie Preißendorfer

Charrier1, Praveen Radhakrishnan1, Claudia Volz1, Thomas SchmidtID1, Alexis Ulrich1,

Sebastian M. Dieter2, Claudia Ball3, Hanno Glimm2,3,4,5, Martin Schneider1

1 Department of General, Visceral and Transplantation Surgery, University of Heidelberg, Heidelberg,

Germany, 2 Translational Functional Cancer Genomics, National Center for Tumor Diseases (NCT)

Heidelberg and German Cancer Research Center (DKFZ), Heidelberg, Germany, 3 Department of

Translational Medical Oncology, National Center for Tumor Diseases (NCT) Dresden and German Cancer

Research Center (DKFZ), Dresden, Germany, 4 Center for Personalized Oncology, University Hospital Carl

Gustav Carus Dresden at TU Dresden, Dresden, Germany, 5 German Consortium for Translational Cancer

Research (DKTK) Dresden, Dresden, Germany

* [email protected]

Abstract

Objectives

Salinomycin is a polyether antibiotic with selective activity against human cancer stem cells.

The impact of salinomycin on patient-derived primary human colorectal cancer cells has not

been investigated so far. Thus, here we aimed to investigate the activity of salinomycin

against tumor initiating cells isolated from patients with colorectal cancer.

Methods

Primary tumor-initiating cells (TIC) isolated from human patients with colorectal liver metas-

tases or from human primary colon carcinoma were exposed to salinomycin and compared

to treatment with 5-FU and oxaliplatin. TICs were injected subcutaneously into NOD/SCID

mice to induce a patient-derived mouse xenograft model of colorectal cancer. Animals were

treated either with salinomycin, FOLFOX regimen, or salinomycin and FOLFOX. Human

colorectal cancer cells were used to delineate an underlying molecular mechanism of salino-

mycin in this tumor entity.

Results

Applying TICs isolated from human patients with colorectal liver metastases or from human

primary colon carcinoma, we demonstrated that salinomycin exerts increased antiprolifera-

tive activity compared to 5-fluorouracil and oxaliplatin treatment. Consistently, salinomycin

alone or in combination with FOLFOX exerts superior antitumor activity compared to FOL-

FOX therapy in a patient-derived mouse xenograft model of colorectal cancer. Salinomycin

induces apoptosis of human colorectal cancer cells, accompanied by accumulation of dys-

functional mitochondria and reactive oxygen species. These effects are associated with

expressional down-regulation of superoxide dismutase-1 (SOD1) in response to salinomy-

cin treatment.

PLOS ONE | https://doi.org/10.1371/journal.pone.0211916 February 14, 2019 1 / 20

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OPEN ACCESS

Citation: Klose J, Trefz S, Wagner T, Steffen L,

Preißendorfer Charrier A, Radhakrishnan P, et al.

(2019) Salinomycin: Anti-tumor activity in a pre-

clinical colorectal cancer model. PLoS ONE 14(2):

e0211916. https://doi.org/10.1371/journal.

pone.0211916

Editor: Joseph Najbauer, University of Pecs

Medical School, HUNGARY

Received: September 25, 2018

Accepted: January 22, 2019

Published: February 14, 2019

Copyright: © 2019 Klose et al. This is an open

access article distributed under the terms of the

Creative Commons Attribution License, which

permits unrestricted use, distribution, and

reproduction in any medium, provided the original

author and source are credited.

Data Availability Statement: All data are within the

paper and its Supplementary Data files.

Funding: This work was supported by DFG grant

KL-2843/2-1 to Johannes Klose and by Deutsche

Krebshilfe (German Cancer Aid) and Colon-Resist-

Net to Claudia Ball and Hanno Glimm. The funder

had no role in study design, data collection and

analysis, decision to publish, or presentation of the

manuscript.

Competing interests: The authors have declared

that no competing interests exist.

http://orcid.org/0000-0002-7289-2891

http://orcid.org/0000-0002-7166-3675

https://doi.org/10.1371/journal.pone.0211916

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Conclusion

Collectively, the results of this pre-clinical study indicate that salinomycin alone or in combi-

nation with 5-fluorouracil and oxaliplatin exerts increased antitumoral activity compared to

common chemotherapy.

Introduction

Colorectal cancer is one of the most common malignancies worldwide with the fourth highest

prevalence among females and males and a lifetime risk of 1 in 20 persons [1,2]. While the

combination of radical surgical resection and neo- and/or adjuvant (radio)chemotherapy

results in 5-year survival rates of 65%, metastasized colorectal cancer is associated with

decreased long-time survival [3]. Colorectal liver metastases are the most common cancer-

related cause of death, leading to 5-year survival rates of less than 15% [2]. Chemotherapy is

fluoropyrimidine-based combined with oxaliplatin or irinotecan and monoclonal antibodies

targeting vascular endothelial growth factor (VEGF) or, in case of no KRAS mutations, endo-

thelial growth factor receptor (EGFR). Effective chemotherapy in the metastasized or palliative

situation is often hindered due to a small fraction of phenotypically different cells within the

primary tumor, which exhibits an increased tumorigenic potential and retains resistance

against chemotherapy and formation of metastases. This subpopulation of cells is commonly

referred to as cancer stem cells [4]. Until today, no specific anti-cancer stem cell therapy exists.

Salinomycin was shown to exhibit selective inhibitory effects against human breast cancer

stem cells [5]. Consequently, activity of salinomycin has been confirmed in numerous types of

cancer, including non-solid malignancies [6], brain [7], bone [8], and lung cancer [9], as well

as gastrointestinal tumors [10–13]. The activity of salinomycin against colorectal cancer cells

has been demonstrated in vitro and in vivo before applying immortalized cell lines [14–16].

Before the initiation of clinical studies, the effectiveness of salinomycin in primary human

colorectal cancer cells has to be demonstrated. For this purpose, tumor-initiating cells (TIC)

represent an attractive source to prove the activity of novel cancer drugs [17]. Isolation and

characterization of TICs derived from human colorectal cancer tissue have been described in

detail before [18–20]. In this pre-clinical experimental study, we investigated the anti-cancer

activity of salinomycin in three TIC cultures derived from colorectal liver metastases and one

TIC culture from a primary colon cancer. The effectiveness of salinomycin was compared to

treatment with a combination of 5-fluorouracil, folic acid, and oxaliplatin, commonly referred

to as FOLFOX regimen. We show in several in vitro-assays that salinomycin exerts anti-stem

cell activity against all four TIC cultures, where FOLFOX therapy exerts only weak effective-

ness. In NOD/SCID mice, patient-derived subcutaneous xenograft models were conducted.

Tumor growth was likewise inhibited by salinomycin treatment.

Results

Exposure to salinomycin reduces the viability of TIC

First, we analyzed the activity of salinomycin in three TIC lines derived from colorectal liver

metastases (Pat 1–3) and one TIC line from colon cancer (Pat 4) using the WST-1 assay.

Each spheroid culture was exposed to increasing concentrations of salinomycin, 5-FU, and

oxaliplatin at equivalent dosages (1, 2, 5, and 10 μM) for 24 and 48 hours. As demonstrated in

Fig 1A–1D, salinomycin significantly and dose-dependently reduced tumor cell viability in all

Salinomycin inhibits human colorectal cancer initiating cells


Abbreviations: Lgr5, leucine-rich-repeat-

containing G-protein-coupled-receptor 5; ROS,

reactive oxygen species; SOD1, superoxide

dismutase 1.


Fig 1. Salinomycin reduces the cell number of colorectal cancer TICs. TIC cultures from patients 1–4 were cultured in the absence

or presence of increasing concentrations of salinomycin, 5-fluorouracil, and oxaliplatin (1, 2, 5, and 10 μM) for 24 and 48 hours. Tumor

cell number was analyzed by WST-1 reduction, which is assumed to be proportional to the cancer cell number. Results are shown as

summary of n = 4 independent experiments as mean ± SEM. � p< 0.05, �� p< 0.001 compared with salinomycin treatment.

https://doi.org/10.1371/journal.pone.0211916.g001





spheroid cultures after 24 and 48 hours of treatment compared to 5-FU. Treatment with oxali-

platin also partially (patients 3 and 4) resulted in reduced TIC viability.

To further corroborate these findings, we applied the CellTiter-Glo Assay to assess the via-

bility of TICs derived from colorectal liver metastases or primary colon carcinoma. Cell

viability of all four spheroid cultures was consistently reduced after exposure to increasing

concentrations of salinomycin (1, 2, 5, and 10 μM) for 24 and 48 hours (S1A–S1D Fig). Salino-

mycin treatment exhibited superior activity against TIC compared to treatment with 5-FU or

oxaliplatin. Of note, exposure of TICs from patient 3 and patient 4 to oxaliplatin also resulted

in decreased cell viability, as already observed in the WST-1 assay.

Salinomycin induces apoptotic death of TICs

To analyze the induction of cell death in TICs isolated from colorectal liver metastases or pri-

mary colon cancer, we examined DNA fragmentation after exposure to salinomycin, or to

5-FU and oxaliplatin (1, 2, 5, and 10 μM) for 24 and 48 hours. The sub G1 population was

regarded as apoptotic cell fraction [21]. A representative histogram of flow-cytometry analysis

is depicted in Fig 2A. As demonstrated in Fig 2B–2E, treatment with salinomycin induced

apoptotic cell death in all spheroid cultures used in this study. Strong pro-apoptotic activity of

salinomycin was observed in TICs derived from patients 1–3. Treatment with 5-FU and oxali-

platin also induced apoptosis in TICs from patients 1 and 2 (Fig 2B and 2C). Oxaliplatin

induced apoptosis in TICs derived from patient 3 after 24 hours as well. In TICs obtained

from patient 4, treatment with 5-FU and oxaliplatin induced apoptosis comparable to salino-

mycin (Fig 2E).

Induction of apoptosis in TICs was also analyzed applying AnnexinV analysis. As shown in

S2B Fig, treatment with salinomycin resulted in induction of apoptosis in a dose-dependent

manner. Consistently with the results obtained in detection of the sub G1 population, induc-

tion of apoptosis was observed after treatment with salinomycin, 5-FU, or oxaliplatin in all

patient-derived TICs (S2B Fig).

Anti-stem cell activity of salinomycin in TIC cultures

Spheroid formation of TICs is regarded as a hallmark of tumorigenic capability and obligatory

to form de novo tumors in immunocompromised mice [17]. To assess the impact of salinomy-

cin on spheroid formation, we exposed all four TIC lines to increasing concentrations of

salinomycin (1, 2, 5, and 10 μM) for 21 days. Strikingly, salinomycin dose-independently

inhibited spheroid formation in all four patient-derived TIC cultures (Fig 3A). In comparison,

after treatment with 5-FU or oxaliplatin, spheroid formation was still detectable even after

exposure to high drug concentrations after three weeks of treatment (S3 and S4 Figs).

Stem cell surface markers like CD133, CD44, or EpCam have been used to enrich colorectal

cancer TICs and to reflect their tumor-initiating properties [22]. Therefore, we investigated

the impact of exposure of TICs to salinomycin (1, 2, 5, and 10 μM) for 24 hours on the surface

expression of CD133, CD44, and EpCam. As demonstrated in S5 Fig, stem cell marker expres-

sion was heterogeneous among the four TIC cultures. While EpCam was consistently

expressed in all cell lines, a high CD133 expression was only observed in TICs derived from

patient 1. TICs from patient 2 revealed only moderate CD133 expression while it was absent in

TICs from patients 3 and 4. CD44 was moderately expressed in all cell lines. Exposure with sal-

inomycin did not alter the stem cell marker expression pattern in all four TIC cultures. Given

that the expression of stem cell surface markers is not predictive for the proliferation or tumor

initiating capacity of the TIC cultures used in this study [19], we did not analyze the impact of

5-FU and oxaliplatin on the stem cell marker expression.




Fig 2. Salinomycin induces apoptosis in colorectal cancer TICs. TIC cultures from patients 1–4 were cultured in the absence or

presence of increasing concentrations of salinomycin, 5-fluorouracil, and oxaliplatin (1, 2, 5, and 10 μM) for 24 and 48 hours.

Induction of apoptotic cell death upon treatment was performed by SubG1 analysis. Data are shown as representative histograms of

flow-cytometric analysis with a logarithmic (A) or linear (S2A Fig) amplification of DNA fluorescence or as summary of n = 3

independent experiments as mean ± SEM (B-E). � p< 0.05, �� p< 0.001 compared with salinomycin treatment.






Fig 3. Anti-stem-cell activity of salinomycin in colorectal cancer TICs. (A) TIC cultures from patients 1–4 were cultured in the

absence or presence of increasing concentrations of salinomycin (1, 2, 5, and 10 μM) for 21 days. Cell morphology and sphere

formation capacity was assessed daily and cell cultures were documented after end of treatment. Results are shown as representative

images (n = 3 individual experiments) of treated TIC with salinomycin (B + C). The activity against cancer stem cells was further

analyzed by measurement of the ALDH1+ population after treatment for 24 and 48 hours. Results are displayed as representative dot

blots (B) or as summary of n = 3 independent experiments as mean ± SEM (C). Analysis of mRNA expression level of Lgr5 after

treatment with increasing concentrations of salinomycin, 5-FU, and oxaliplatin for 24 hours was further investigated and shown as

summary of n = 3 independent experiments as mean ± SEM (D); � p< 0.05 and �� p< 0.001 compared with salinomycin treatment.

Scale bars = 100 μM.






Aldehyde dehydrogenase-1 (ALDH1) is regarded as a marker to label colorectal cancer

stem cells [23]. Its relevance for the tumorigenity of the TIC cultures used in this study has not

been investigated so far. Therefore, we assessed ALDH1 expression after treatment with salino-

mycin, 5-FU, and oxaliplatin for 24 and 48 hours. Exposure of all TIC lines to salinomycin

resulted in reduction of the ALDH1+ population, particularly after treatment for 48 hours. Of

note, ALDH1+ reduction was less pronounced in TICs derived from patient 1 (Fig 3B+3C).

Strikingly, exposure to 5-FU and oxaliplatin resulted in a more pronounced reduction of the

ALDH1+ population compared to salinomycin treatment. This effect was observed in all four

TIC cultures (Fig 3C).

We further investigated the anti-stem cell activity of salinomycin, 5-FU, and oxaliplatin in

patient-derived TICs applying qPCR to analyze the mRNA expression of Lgr5, which is

regarded as substantial contribution to the colorectal cancer stem cell hierarchy and to colorec-

tal carcinogenesis [24]. As demonstrated in Fig 3D, salinomycin-exposure for 24 hours

resulted in a dose-dependent reduction of Lgr5 mRNA expression in all four TIC lines. Expo-

sure to increasing concentrations of 5-FU or oxaliplatin did also alter Lgr5 mRNA expression

in TICs. Of note, the suppressive effect on Lgr5 mRNA expression was more pronounced after

treatment with salinomycin in comparison to common chemotherapeuticals. TICs derived

from patient 1 did not react in the same manner compared to TICs derived from patients 2–4

(Fig 3D).

Salinomycin inhibits tumor growth in a patient-derived xenograft model

Next, we applied a patient-derived xenograft model to investigate the effects of salinomycin on

colorectal cancer growth in vivo. TICs were injected subcutaneously into the flank of NOD/

SCID-IL2RGnull mice. After successful tumor formation, animals were treated either with vehi-

cle, salinomycin, or FOLFOX (5-FU, folic acid, and oxaliplatin (Fig 4A). Chemotherapy was

tolerated by the animals as demonstrated by analysis of body weight during treatment (S6 Fig).

Treatment with salinomycin was superior compared to FOLFOX in one patient-derived

xenograft model (patient 1) and equivalent to FOLFOX in two xenograft models (patients 3

and 4). FOLFOX therapy was superior compared to salinomycin in the xenograft model

derived from patient 1 (Fig 4B). Combined treatment with salinomycin and FOLFOX resulted

in increased anti-tumoral activity compared to FOLFOX alone in all xenograft models. The

growth-inhibiting effect of the combination therapy was statistically significant in vivo in TICs

derived from patients 2, 3, and 4. Representative images of H&E stained tumor sections and

explanted tumors are depicted in Fig 4C+4D.

Salinomycin inhibits proliferation, induces cell death and abolishes ATP

production of human colorectal cancer cells

To delineate the molecular mechanism underlying salinomycin’s potency against human colo-

rectal cancer cells, we assessed the impact of salinomycin on mitochondrial function in three

human colorectal cancer cell lines. We hypothesized that salinomycin interferes with mito-

chondrial function leading to impaired tumor cell survival.

At the outset, we investigated whether treatment with salinomycin inhibits proliferation

and survival of three human colorectal cancer cell lines. For this purpose, HT29, SW480, and

HCT116 cells were exposed to increasing concentrations of salinomycin (0.1, 0.5, 2, 5, and

10 μM) for 24 hours, and tumor cell proliferation was assessed using the BrdU incorporation

assay. As demonstrated in S7A Fig, treatment with salinomycin resulted in a dose-dependent

inhibition of tumor cell proliferation.




Fig 4. Salinomycin inhibits tumor growth in a patient-derived xenograft model. (A) Subcutaneous colorectal cancer growth in NOD/

SCID-IL2RGnull mice was induced through injection of TICs into the right flank of the animals. After tumor growth, treatment with either

vehicle, salinomycin, or FOLFOX regimen (5-fluorouracil, calcium folinate, and oxaliplatin) was started. (B) After 21 days of treatment,

Salinomycin significantly inhibited colorectal cancer growth in the subcutaneous tumor model of patient 2 and was non-inferior in the

tumor models of patients 3 and 4. In contrast, FOLFOX therapy was superior compared to salinomycin in the tumor model of patient 1.

Combined treatment with salinomycin and FOLFOX resulted in increased anti-tumoral activity in all tumor models. Results are shown as

mean tumor volume ± SD (C), H&E stained sections (D), and representative images of explanted tumors of 7 individual experiments. Scale

bars = 100 μM.






Next, we analyzed induction of cell death following exposure to salinomycin and measured

LDH release in HT29, SW480, and HCT116 cells after 24 hours of treatment. As shown in S7B

Fig, salinomycin causes increased LDH release in all three cell lines. We further analyzed

induction of apoptosis by measuring the amount of AnnexinV/PI positive cells. Treatment

with salinomycin likewise resulted in a dose-dependent induction of apoptosis in all cell lines

(S7C Fig).

After confirming the cytotoxic activity of salinomycin in colorectal cancer cells, we investi-

gated the impact of salinomycin on cellular ATP production. As shown in S7D Fig, treatment

with increasing concentrations of the compound was associated with decreased cellular ATP

levels. Cell viability in these experiments was monitored in parallel (S8 Fig).

Accumulation of dysfunctional mitochondria and increased production of

reactive oxygen species upon salinomycin treatment

Based on the observation that reduced ATP levels of tumor cells might be associated with an

increased amount of dysfunctional mitochondria (and consequently increased production of

reactive oxygen species (ROS) [11]), we analyzed whether treatment with salinomycin results

in accumulation of mitochondrial mass. As shown in Fig 5A+5B, treatment with salinomycin

for 24 hours results in an increased amount of mitochondrial mass in colorectal cancer cells.

Given that accumulation of mitochondrial mass is an indicator of mitochondrial dysfunction,

we analyzed the generation of ROS after exposure to salinomycin. Indeed, treatment with sali-

nomycin for 24 hours led to increased ROS generation in HT29, SW480, and HCT116 cells

(Fig 5C+5D).

Exposure to salinomycin inhibits respiratory chain complex II and

increases SOD1 expression

Finally, we aimed to investigate whether treatment with salinomycin is related to inhibitory

effects on the mitochondrial respiratory chain. Therefore, we analyzed the activity of respira-

tory chain complexes I, II, and citrate synthase activity. Exposure to salinomycin resulted in

inhibition of complex II in all three colorectal cancer cell lines. Function of complex I and cit-

rate synthase activity were unaffected by salinomycin treatment (S9A–S9C Fig).

To correlate these findings with genomic expression patterns of ROS- and cellular repair-

related genes, we performed qPCR analysis and analyzed the mRNA expression of superoxide

dismutase 1 (SOD1). SOD1 expression is postulated to protect cells from damage caused by

ROS [25,26]. As shown in S9D Fig, mRNA expression of SOD1 was strikingly inhibited by sali-

nomycin treatment in a dose-dependent manner.

Discussion

The basic principles of systemic chemotherapy include classical cytostatic drugs, antibody-,

hormone- or immuno-based therapies, and, as of late, targeted therapies. A paradigm shift in

basic oncological research relates to the discovery of salinomycin as a specific inhibitor of can-

cer stem cells [27–30]. This could include treating primary tumor growth, the formation of

metastases, and tumor recurrence. The aim of the present pre-clinical study was to investigate

the impact of salinomycin on patient-derived colorectal cancer initiating cells. The results con-

firm that salinomycin exhibits increased anti-cancer stem cell activity compared to 5-FU and

oxaliplatin in vitro. Furthermore, salinomycin inhibits tumor growth in a patient-derived colo-

rectal cancer xenograft model. Combined treatment with salinomycin, 5-FU, and oxaliplatin

resulted in superior activity compared to salinomycin monotherapy.




Fig 5. Treatment with salinomycin results in accumulation of dysfunctional mitochondria and increased generation of ROS.

(A) Analysis of total mitochondrial mass in HT29, SW480, and HCT116 cells after exposure to increasing concentrations of

salinomycin (0.1, 0.5, 2, 5, and 10 μM) was assessed using MTR green. After 24 hours, accumulation of mitochondrial mass was

observed in all three cell lines. (B) Accumulation of dysfunctional mitochondria was further visualized applying MRT green

immunostaining after 24 hours of treatment. (C) Increased generation of ROS in HT29, SW480, and HCT116 cells after treatment

with salinomycin using CM-H2DCFDA staining and analyzed by flow cytometry. (D) Immunostaining of HT29, SW480, and




Salinomycin, initially isolated from Streptomyces albus [31], was used over decades in ani-

mal farming due to its coccidiostatic activity [32]. Gupta et al. described in 2009 for the first

time the inhibitory effects of the compound in breast cancer stem cells [5]. Thereupon, the

anti-cancer activity of salinomycin has been studied in detail over the past years [28]. Until

now there are no data available analyzing the activity of salinomycin in patient-derived cancer

stem cells. For colorectal cancer, the stem-cell specific activity of salinomycin was extrapolated

from stem-like cells within immortalized human colorectal cancer cell lines [14–16,33–36].

Therefore, we investigated the activity of salinomycin against three TIC lines derived from

patients with colorectal liver metastases and one TIC line derived from a patient with colon

cancer in vitro and in vivo. The obtained findings indicate that salinomycin might display

potential for the treatment of colorectal cancer in (pre)clinical practice.

First, we demonstrate that salinomycin reduces viability and inhibits proliferation of all

TIC cultures in a dose-dependent manner, whereas 5-FU and partially oxaliplatin stay less

active. Furthermore, salinomycin induces apoptosis in TIC cultures in a time-dependent man-

ner. This effect has been observed in colorectal cancer cells and other types of cancer before

[10,11,34,37]. Strikingly, salinomycin acts directly inhibitory of cancer stem cells by inhibition

of spheroid formation and transcript levels of stem cell-related genes. Particularly, the

decreased expression of Lgr5, one of the most important genes for stem cell hierarchy and

maintenance in colorectal cancer, underlines the stem cell activity of salinomycin [24,38]. Fur-

thermore, Lgr5 is required for the maintenance of spheroid-derived colorectal cancer cells

[39]. The expression of cancer stem cell surface markers, such as CD133, CD44, or EpCam,

was not influenced by salinomycin treatment. However, the tumorigenic potential of the

TICs used in this study is independent of the expression of stem cell surface markers [18,19].

ALDH1 expression, which has been demonstrated to label colorectal cancer stem cells [23], is

also inhibited by salinomycin treatment. Of note, exposure to 5-FU and oxaliplatin did also

reduce ALDH1 expression in TICs used in this study.

Second, salinomycin inhibits tumor growth in four patient-derived colorectal cancer xeno-

graft models. Compared to FOLFOX regimen, treatment with salinomycin was superior

regarding inhibition of tumor growth in one model and non-inferior in two models. In the

xenograft model derived from patient 1, FOLFOX therapy was superior compared to treat-

ment with salinomycin. This inhomogeneous response to salinomycin treatment also reflects

the individuality of each tumor. Strikingly, the combination of salinomycin and FOLFOX

resulted in increased anti-tumoral activity in all four patient-derived xenograft models. This

synergistic effect of salinomycin and FOLFOX might represent the backbone for further pre-

clinical investigations. Synergistic effects of salinomycin and other established cytotoxic drugs

have been described before [40–42].

The heterogeneous response of salinomycin treatment among the four patient-derived

TICs in vitro and in vivo is explainable by the biological heterogeneity of each tumor and

reflects the clinical reality. Similar observations have been made before when the anti-cancer

activity of several drugs in patient-derived xenografts were investigated [43–45].

Third, induction of apoptosis by salinomycin is associated with accumulation of dysfunc-

tional mitochondria and ROS, and decreased cellular ATP production in human colorectal

cancer cells. It is assumed that cancer cells are dependent on functional mitochondria to main-

tain their cellular energy production [46]. Indeed, sufficient ATP production as a source for

HCT116 cells after exposure of salinomycin confirmed increased ROS generation. Results are displayed as a summary of at least

three independent experiments as mean ± SD or as representative image capture by fluorescence microscopy; � p< 0.05 compared

with control. N = 3 individual experiments. Scale bars = 50 μM.






cellular energy is regarded as mandatory for epithelial cancer cells to show migration activity

and to form distant metastasis [47]. Furthermore, salinomycin inhibits the activity of complex

II, which is a key enzyme in mitochondrial respiration. The accumulation of ROS in colorectal

cancer cells might be explained by decreased expression of SOD1, which is assumed to exert

protective effects on cells damaged by ROS [25]. In line, inhibition of SOD1 in mice was

shown to inhibit angiogenesis and proliferation, making it a potential target of anti-cancer

therapies [48].

Of note, we do not imply that inhibition of SOD1 might represent an exclusive mechanism

of salinomycin to treat colorectal cancer cells. Inhibition of Wnt-signalling or interference

with autophagic flux might also contribute to the mode of action of salinomycin [15,34]. In

breast cancer stem cells, salinomycin acts via accumulation in the endoplasmatic reticulum

leading to enhanced Ca2+ release into the cytosol, which is the initiating event for inhibition of

the Wnt signalling pathway [49]. In osteosarcoma and primary breast cancer cells, salinomycin

eliminates cancer stem cells by sequestering iron in lysosomes [50]. Thus, the observed effects

in this study might be a consequence of the Ca2+ increase in the cytoplasm induced by salino-

mycin accumulation in the endoplasmatic reticulum. Detailed genetic characterization of the

patient-derived xenograft model used in this study will provide further insights into the mech-

anisms of action of salinomycin in colorectal cancer stem cells.

Only rare data exist investigating the pre-clinical usage of salinomycin in men. One case-

report describes intravenous administration of salinomycin in a small cohort of patients with

metastasized breast, ovarian, and head and neck cancer. After three weeks of treatment, partial

tumor and metastases regression has been observed. Interestingly, only minor acute or long-

term side effects are described [51]. Despite all justifiable expectations of salinomycin and its

clinical applicability to treat colorectal cancer, the potential toxic side effects of the drug cannot

be neglected. Accidental severe intoxications in humans and animals have been described

[52,53]. Synthesis of structural analogs with an improved activity and reduced toxicity might

be an alternative to the original compound [54–56]. We have demonstrated its effectiveness

against colorectal cancer cells recently [37]. Thus, semi-synthetic analogs of salinomycin used

at lower concentrations show equivalent activity against colorectal cancer cells than salinomy-

cin, which may have potential for reducing the toxic side effects of salinomycin.

Conclusions

In summary, the results of this study demonstrate the activity of salinomycin in a patient-

derived pre-clinical model for colorectal cancer in vitro and in vivo. Thus, salinomycin remains

a candidate for the (pre)clinical usage to treat colorectal cancer. Combined treatment with sali-

nomycin and FOLFOX might be the best applicability. Further studies are needed to discover

the best drug dose-intensity and potential toxic effects in normal, non-cancerous cells.

Methods

Cell lines and culture

We used four TIC spheroid cultures obtained from patients with metastasized colorectal can-

cer in accordance with the Declaration of Helsinki and after approval of the Ethics Commis-

sion of the University Hospital of Heidelberg. Informed consent was obtained from all

patients. Patient characteristics are summarized in S1 Table. Establishment and growth of

spheroid cultures was described extensively before [18,19]. In brief, tumor tissue was minced

and enzymatically digested with dispase and cultured in ultra-low attachment flasks (Corning).

This results in the death of non-cancerous cells and formation of 3D spheroid cells. Cell cul-

ture was continued under serum-free conditions in advanced D-MEM/F-12 medium




supplemented with glucose to 0.6%, 1% penicillin/streptomycin, 1 mM L-glutamine (Invitro-

gen), 4 μg/ml heparin, 5 mM HEPES, 4 mg/ml BSA (Sigma-Aldrich), 10 ng/ml fibroblast

growth factor (FGF) basic, and 20 ng/ml epidermal growth factor (EGF; R&D Systems).

Growth factors were added every 4 days and medium was changed every 7 to 14 days [18]. The

cells were incubated at 37 ˚C and 5% CO2. Authentication of the spheroid cultures and exclu-

sion of contamination was performed by Multiplexion (http://www.multiplexion.de/en/). For

immunophenotyping, the cells were dissociated with accutase and stained with anti-human

CD133 (clone AC133, Miltenyi), anti-human CD44 (clone G44-26, Becton Dickinson), and

anti-human CD326 (EpCam, clone 9C4, BioLegend). Surface marker expression was analyzed

by flow-cytometry (FACSCalibur, Becton Dickinson). In this study, the same spheroid cultures

were used as described and characterized in detail before [18,19]. In all experiments, TICs

were used and plated as spheres. Therefore, TICs were dissociated with accutase, stained with

Trypan Blue solution and counted using Neubauer chambers. In all experiments with TICs,

non-adherent cell culture material was used.

The three human CRC cell lines HT29, SW480, and HCT116 were used to investigate the

molecular mechanism of salinomycin in vitro. Cells were obtained from American Type Cul-

ture Collection (ATCC numbers: HTB38, CCL-228, and CCL-247). SW480 and HT29 cells

were cultured in RPMI 1640 medium (Invitrogen); HCT116 cells were cultured in DMEM

High Glucose medium (Sigma Aldrich). All media were supplemented with 10% fetal calf

serum, penicillin (50 U/ml) and streptomycin (50 mg/l). The cells were incubated at 37 ˚C and

10% CO2.

Chemicals and antibodies

Salinomycin sodium salt was purchased from Sigma Aldrich and dissolved in dimethyl sulfox-

ide (DMSO) to obtain a stock solution (10 mM). The DMSO concentration in the medium

was 0.05% when using the compound at a 0.5 μM concentration. For the experiments, Salino-

mycin-Na (sodium salt) has been used, abbreviated as “salinomycin”.

5-fluorouracil (5-FU), calcium folinate, and oxaliplatin were obtained from the pharmacy

of the University Hospital of Heidelberg. All compounds were diluted with phosphate-buff-

ered saline (PBS) to receive appropriate working solutions [10,57]. Stock solutions were stored

at -20 ˚C.

Cell proliferation and viability assay

TICs or the three human colorectal cancer cell lines HT29, SW480, and HCT116 (5 x 103 each)

were cultured in 96-well flat bottom plates. Cells were exposed to increasing concentrations of

salinomycin, 5-FU, or oxaliplatin (0.1 μM, 0.5 μM, 1 μM, 2 μM, 5 μM, and 10 μM) for 24–48

hours. The effect of the evaluated compounds on the cell number was assessed using the WST-

1 assay (Sigma Aldrich) according to the manufaturer’s instructions. Cell proliferation was

assessed using the bromodeoxyuridine (BrdU) ELISA kit (Sigma Aldrich), according to the

manufacturer’s instructions. Additionally, cell numbers were analyzed applying the CellTiter-

Glo Viability Assay (Promega). This assay is based on measuring a luminescence signal pro-

portional to the ATP content present in healthy cells. The luminescence signal was quantified

by a microplate luminometer as recommended by the manufacturer’s instructions.

Cell death

Induction of TICs’ death after exposure to salinomycin, 5-FU, or oxalipaltin was analyzed

measuring DNA fragmentation. 0.5x106 cells were seeded in 6-well plates and exposed to



http://www.multiplexion.de/en/


increasing concentrations of salinomycin, 5-FU, or oxaliplatin (1 μM, 2 μM, 5 μM, and 10 μM)

for 24 and 48 hours. After permeabilisation, the cells were stained with the DNA intercalating

dye propidium iodide (PI). The DNA profile was measured by flow-cytometry and the apopto-

tic cells were defined as the sub G1 fraction as previously described [21]. During analysis,

gates were set to exclude small particles with low DNA content. To receive a better resolution

between the two peaks in cells growing in suspension, analysis was performed with a logarith-

mic amplification of DNA fluorescence [21]. Induction of apoptosis of TICs was further

assessed by AnnexinV analysis (BD Biosciences). TICs were treated either with increasing con-

centrations of salinomycin, 5-FU, or oxaliplatin (1 μM, 2 μM, 5 μM, and 10 μM) for 24 hours.

Cells were dissociated with accutase and stained according to the manufacturers’ instructions.

Induction of apoptosis was measured by flow-cytometry.

Alternatively, cell death of HT29, SW480, and HCT116 cells was further analyzed applying

the Lactate dehydrogenase (LDH) Cytotoxicity Assay Kit (Thermo Fisher) following the man-

ufacturer’s instructions as previously described [37].

Spheroid formation assay

Spheroid formation assay was conducted to assess the long-lasting effects of salinomycin com-

pared to FOLFOX treatment on the four TIC cultures used in this study. Therefore, TIC

spheres were dissociated with accutase and 2x104 cells were seeded in a 24-well plate. Cells

were treated with increasing concentrations of salinomycin, 5-FU, or oxaliplatin (1 μM, 2 μM,

5 μM, and 10 μM) and observed daily. Medium was not changed, but growth factors were

added every three days. Spheroid formation was assessed after 21 days using phase-contrast

microscopy.

Cancer stem cell activity assay

The anti-cancer stem cell activity of salinomycin was analyzed applying the ADELFOUR kit

(Stem Cell Technologies) according to the manufacturer’s protocol. After exposure to salino-

mycin, 2x105 TICs were either incubated with the ALDH1 inhibitor diethylaminobenzalde-

hyde (DEAB) and the ALDH1 substrate BODIPY-amino acetaldehyde or only BODIPY-

amino acetaldehyde. DEAB-treated cells served as a control to set the ALDH1+ region for each

sample using a Becton Dickinson flow cytometer as described before [58].

RNA isolation and real-time PCR

Total RNA from tumor cells was isolated by an RNA extraction kit (Qiagen) and cDNA

synthesis and real-time (RT)-PCR were performed using the first strand cDNA synthesis kit

(Fermentas) and SYBR Green Master Mix kit (Roche) applying specific primers (Life Technol-

ogies) for human leucine-rich-repeat-containing G-protein-coupled-receptor 5 (Lgr5) and

superoxide dismutase 1 (SOD1). Transcript levels of the gene of interest were normalized to

the expression of glyceraldehy-3-phosphat-dehydrogenase (GAPDH). Primer sequences are

listed in S2 Table.

Analysis of ATP production

Cellular ATP was quantified using a luciferase-based assay (Promega) according to the manu-

facturer’s protocol. Cell viability in these experiments was monitored in parallel using the

WST-1 assay as described above.




Mitochondrial assays and measurement of ROS generation

Mitochondrial mass was analyzed by flow cytometry or fluorescence microscopy using Mito-

Tracker Green FM (MTR green) staining (Molecular Probes, Eugene, OR, USA), according to

the manufacturer’s instructions and described in [10]. Complex I and complex II activity was

measured using the Complex I and II Enzyme Activity Microplate Assay Kits (Abcam) accord-

ing to the manufacturer’s protocols and described in [59]. Citrate synthase activity was mea-

sured using the Citrate Synthase Assay Kit (Sigma).

The ROS generation was determined by flow cytometry, applying 5-(and-6)-chloromethyl-

20,70-dichlorodihydrofluorescein diacetate, acetyl ester (CM-H2DCFDA) staining (Invitrogen)

according to the manufacturer’s instructions and described in [10]. The ROS generation was

further analyzed by applying the MitoTracker Red CM- H2-Xros Kit (Invitrogen) according to

the manufacturer’s instructions.

Animal model and treatment

Animal experiments were carried out in 6–10 week-old NOD/SCID-IL2RGnull mice purchased

from Charles River Laboratories. Animals were housed under specific-pathogen-free condi-

tions in groups of four with free access to food and water under constant environmental condi-

tions with a 12-hour day-night-cycle. Isoflurane was used for inhalation anesthesia. At the

start of the experiments, animals weighed 26.7 g ± 1.4 (mean ±SD). For the patient-derived

xenograft model, 1 x 106 TICs were injected in 50 μl Matrigel (BD Biosciences) into the right

flank under inhalation anaesthesia. Following successful tumor formation after 6–8 weeks, ani-

mals were randomized into four treatment groups containing seven mice each. The animals

were treated daily at 09:00 a.m. in the home cage either with corn oil (control group), 4 mg/kg

salinomycin, FOLFOX (8 mg/kg 5-FU + 20 mg/kg calcium folinate (on six consecutive days)

and 10 mg/kg oxaliplatin (once a week)) intraperitoneally, or a combination of salinomycin

and FOLFOX [60]. Injection of chemotherapy was performed without anaesthesia. Animal

health was monitored twice a day daily. Animals were weighed every second day. Humane

endpoints were defined as tumor size > than 15 mm in diameter, tumor ulceration, reduced

activity of the animal, weight loss > than 20% of the original weight, reduced softness of the

coat, and abnormal behavior. No adverse events were observed. Tumor volume was assessed

daily during chemotherapy for a total of 21 days. The maximum tumor size achieved during

the study was 13.9 x 14.9 mm. Euthanasia was performed by cervical dislocation. The local ani-

mal care committee (Regierungsprasidium Karlsruhe) approved all experiments. The experi-

ments were carried out in accordance with the criteria outlined in the “Guide for the Care and

Use of Laboratory Animals” by the National Academy of Sciences. All efforts were made to

minimize suffering.

Immunohistochemistry

Fixed, paraffin-embedded tissue samples were cut into sections of 5 μm and routine hematoxy-

lin and eosin (H&E) staining was performed to evaluate histomorphological features.

Statistical analysis

Statistical analysis was performed using GraphPadPrism 6. Student’s t-test or ANOVA analysis

were applied as appropriate.

Differences were regarded statistically significant with p<0.05 compared to untreated (indi-

cated as “control”) or treated cells. Results were expressed as mean ± standard error of the

mean (SEM) of at least three independent experiments.




All data are within the paper and its Supplementary Data files.

Supporting information

S1 Fig. Salinomycin reduces viability of colorectal cancer TICs. TIC cultures from

patients1-4 were cultured in the absence or presence of increasing concentrations of salinomy-

cin, 5-fluorouracil, and oxaliplatin (1, 2, 5, and 10 μM) for 24 and 48 hours. Tumor cell viabil-

ity was assessed applying the CellTiter-Glo Viability Assay. Results are shown as summary of

n = 4 independent experiments as mean ± SEM. � p< 0.05, �� p < 0.001 compared with sali-

nomycin treatment.

(TIF)

S2 Fig. Induction of apoptosis in TICs after exposure to salinomycin, 5-FU, and oxalipla-

tin. TIC derived from patients1-4 were cultured in the absence or presence of increasing con-

centrations of salinomycin, 5-fluorouracil, and oxaliplatin (1, 2, 5, and 10 μM) for 24 hours.

Induction if apoptosis was analyzed using SubG1 or AnnexinV-FITC and PI staining and cells

were analyzed by flowcytometry. Results are shown as linear amplification of DNA fluores-

cence (A) or as summary of n = 3 independent experiments as mean ± SEM (B). � p< 0.05,�� p< 0.001 compared with salinomycin treatment.

(TIF)

S3 Fig. Preserved spheroid formation of TICs after exposure to 5-fluorouracil. TIC cultures

from patients1-4 were cultured in the absence or presence of increasing concentrations of

5-fluorouracil (5-FU; 1, 2, 5, and 10 μM) for 21 days. Cell morphology and sphere formation

capacity was assessed daily and cell cultures were documented after end of treatment. Results

are shown as representative images (n = 3 individual experiments) of treated TIC with salino-

mycin. Scale bars = 100 μM.

(TIFF)

S4 Fig. Preserved spheroid formation of TICs after exposure to oxaliplatin. TIC cultures

from patients1-4 were cultured in the absence or presence of increasing concentrations of oxa-

liplatin (Oxa; 1, 2, 5, and 10 μM) for 21 days. Cell morphology and sphere formation capacity

was assessed daily and cell cultures were documented after end of treatment. Results are

shown as representative images (n = 3 individual experiments) of treated TIC with salinomy-

cin. Scale bars = 100 μM.

(TIFF)

S5 Fig. Impact of Salinomycin on stem cell marker surface expression of colorectal cancer-

derived TICs. Colorectal cancer-derived TICs were exposed to salinomycin (1, 2, 5, and

10 μM) for 24 hours. Expression of the stem cell surface markers CD133, CD44, and EpCam

were analyzed by flow-cytometry. Results are shown as representative images (n = 3 individual

experiments) of treated TIC with salinomycin.

(TIFF)

S6 Fig. Body weight of the animals after treatment. Effect of Salinomycin treatment on body

weight (g) of mice in each group.

(TIFF)

S7 Fig. Salinomycin inhibits proliferation, induces cell death and reduces ATP levels in

human colorectal cancer cell lines. HT29, SW480, and HCT116 cells were cultured in in the

absence or presence of increasing concentrations of salinomycin (0.1, 0.5, 2, 5, and 10 μM) for

24 hours. Tumor cell proliferation was assessed using the BrdU incorporation assay (A). Cell



http://www.plosone.org/article/fetchSingleRepresentation.action?uri=info:doi/10.1371/journal.pone.0211916.s001








death was determined by LDH release assay (B). Induction if apoptosis was analyzed using

AnnexinV-FITC and PI staining and cells analyzed by flowcytometry (C). Intracellular ATP

levels were assessed applying a luciferase-based ATP assay (D). Results are displayed as a sum-

mary of n = 3 independent experiments as mean ± SD; � p< 0.05 compared with control.

(TIFF)

S8 Fig. Monitoring of cell viability during analysis of cellular ATP levels. Cell viability

during analysis of cellular ATP levels was monitored using the WST-1 assay in parallel. Results

are displayed as a summary of n = 3 independent experiments as mean ± SD; � p< 0.05,�� p< 0.001 compared with control.

(TIFF)

S9 Fig. Salinomycin inhibits activity of complex II and reduces the mRNA expression of

SOD1. Analysis of complex I (A), II (B), and citrate synthase activity (C) after exposure of

HT29, SW480, and HCT116 cells after treatment with 2 and 10 μM salinomycin for 24 hours.

mRNA expression of SOD1 in HT29, SW480, and HCT116 cells after exposure to increasing

concentrations of salinomycin (0.1, 0.5, 2, 5, and 10 μM) for 24 hours was measured by

qRT-PCR. Results are displayed as a summary of n = 3 independent experiments as

mean ± SD; � p< 0.05, �� p< 0.001 compared with control.

(TIFF)

S1 Table. Patient characteristics.

(TIFF)

S2 Table. Primer sequences of human GAPD, Lgr5, and SOD1.

(TIFF)

Acknowledgments

The authors are thankful to Marzena Knyssok-Sypniewski for her great technical support.

Author Contributions

Conceptualization: Johannes Klose, Praveen Radhakrishnan, Sebastian M. Dieter, Martin

Schneider.

Data curation: Johannes Klose, Martin Schneider.

Formal analysis: Johannes Klose, Thomas Schmidt, Alexis Ulrich, Claudia Ball, Martin

Schneider.

Funding acquisition: Johannes Klose.

Investigation: Johannes Klose, Stefan Trefz, Tobias Wagner, Luca Steffen, Arsalie Pre-

ißendorfer Charrier, Praveen Radhakrishnan, Claudia Volz.

Methodology: Johannes Klose, Thomas Schmidt, Sebastian M. Dieter, Hanno Glimm, Martin

Schneider.

Project administration: Johannes Klose, Claudia Ball.

Resources: Johannes Klose.

Software: Johannes Klose.

Supervision: Johannes Klose, Praveen Radhakrishnan, Hanno Glimm, Martin Schneider.








Validation: Johannes Klose, Alexis Ulrich, Hanno Glimm.

Visualization: Johannes Klose.

Writing – original draft: Johannes Klose.

Writing – review & editing: Johannes Klose, Martin Schneider.

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